1. Field of the Invention
The present invention relates to an electric screwdriver and a controller thereof.
2. Description of the Related Art
There has been provided one type of electric screwdriver equipped with a controller having a function for preventing any screws from being left not tightened. The electric screwdriver, in this type, is connected to the controller, so that when a tightening torque has reached a value equal to or greater than a predetermined value in a tightening operation, a signal indicating the completion of screw-tightening is sent from the electric screwdriver to the controller, and then the controller causes a specific counter (a counter for counting a number of screws tightened) to increment by one to provide a count indication of the number of tightened screws or inform a situation that the number of tightened screws has reached a predetermined one (see, for example, Japanese Patent Application Publication No. 2000-47705, Japanese Patent Application Publication No. Hei7-80128 and Japanese Patent Application Publication No. 2003-123050.
In the tightening operation using the electric screwdriver, however, there might be an event that a screw has been tightened out of upright. If a screw is tightened out of upright, typically the tightening torque could reach the predetermined value in a shorter time than in a tightening operation normally carried out. Also, when the screw that has been once tightened is tightened again (for additional tightening), the tightening torque could reach the predetermined value in a shorter time than in a tightening operation normally carried out. Due to this fact, a typical (in most cases, minimal) tightening time expected in the normal tightening operation is previously set as a reference time in the controller, such that if the signal indicating the completion of the tightening is generated by the electric screwdriver in a shorter time than the reference time, then the controller can determine that the screw must have been tightened in the inclined state or a once tightened screw has been tightened again (for additional tightening).
To determine the aforementioned reference time, it is conventional that a test of tightening is carried out on a certain number of screws prior to a practical tightening operation, in which a time consumed for properly operating screw-tightening is measured by using, for example, a stopwatch, and specifically a shortest time in the time measurements is taken as the reference time. Such a tightening test is, however, a highly cumbersome work for a user.
There is another possible event in the practical operation, a screw might be tightened at an incorrect location, and in that case, an operation (manipulation) for inversely turning the electric screwdriver is carried out to untighten the screw. In such a case, in response to the operation for inversely turning the electric screwdriver, a counter for counting a number of tightened screws is needed to be reset so as to cancel the count in the incorrect tightening operation. Regarding this point, a device according to the prior art has following problems.
FIG. 9 shows a typical wiring diagram in an electric screwdriver D comprising a controller P with a function for preventing any screws from being left not tightened. In the drawing, “D1” and “D2” designate power source lines to a screwdriver motor M, “D3” is an start/stop signal line, “D4” is a tightening completion signal line and “D5” is a common line. The common line is connected to a reference voltage Vs. The power source lines D1 and D2 are connected to a power supply circuit S of the controller P. The screwdriver motor M is a type of direct-current motor and the power circuit S comprises a direct-current power source. “RS” designates a normal/reverse rotation select switch for changing the polarity of the power source output voltage in order to select the rotation direction of the motor M (normal or reverse rotation). “SS” designates a starting switch, which in response to an open/close operation by an operator, sends a start/stop signal to the power circuit S via the start/stop signal line D3. For example, when the starting switch SS is closed, the reference voltage Vs from the common line D5 is applied to the power circuit S as the start signal, whereas when the starting switch is opened, the reference voltage Vs from the common line D5 is shut off, and the stop signal is sent to the power supply circuit by the shut-off. “BS” designates a braking switch of a normally open type, which is shifted to a closed position by an action of a cam C incorporated in the screwdriver when the load torque in the tightening operation has reached a predetermined value. When the braking switch BS is closed, a tightening completion signal at the reference voltage Vs is sent to a counter T for counting the number of tightened screws and also to the power supply circuit S in the controller P via the tightening completion signal line D4.
The circuit shown in FIG. 9 is not able to provide the signal from the electric driver D to the controller P to reverse the count in the counter T in a micro computer A for counting the number of the tightened screws when the user drives the electric screwdriver D in reverse direction to untighten the screw that has been tightened incorrectly as discussed above. To address this, it may be contemplated that the circuit configuration of the electric screwdriver D is modified as such as shown in FIG. 10. Specifically, the normal/reverse rotation selection switch RS is moved from the motor power source lines D1 and D2 to signal line D5 and replaced by an ON/OFF switch, wherein the power source circuit in the controller P may be provided with normal/reverse changing function. In this modification, to cause the electric screwdriver D to rotate in reverse direction, the normal/reverse rotation selection switch RS may be turned to ON position (closed state), for example, so as to send a signal (ON signal) of the reference voltage Vs from the common line D6 to the power supply circuit S in the controller P through the signal line D5 as a reverse rotation control signal, and to cause the power supply circuit S to invert the polarity of the output voltage (motor driving voltage), while at the same time sending the same ON signal to the counter T to reverse the count back.
However, such a configuration would lead to some disadvantages, including a problem of cost increase in association with the increased number of signal lines in the electric screwdriver and another problem of no compatibility that an existing electric screwdriver is no more applicable to the controller having been modified as such as described above.